Conventionally, in a flow rate regulating valves, the valve opening thereof is detected using an angle sensor, where the valve opening is sent to a positioner main unit, which is the valve opening controlling device, to calculate, within this positioner main unit, a process variable in accordance with the difference between a valve opening setting value, applied from the outside, and the detected valve opening, to perform adjustments automatically in accordance with the calculated process variable, to cause the valve opening of the flow rate regulating valve to match the valve opening setting value.
FIG. 7 is a structural diagram of a conventional system for controlling a flow rate regulating valve (a flow rate regulating valve controlling system), where, in this figure, 1 is the flow rate regulating valve, 2 is an operating device, 3 is a yoke for securing the operating device 2 and the flow rate regulating valve 1, 4 is a valve driving shaft, driven by the operating device 2, and 5 is a pin that is embedded protruding at a prescribed position of the valve driving shaft 4. 6 is an angle sensor (VTD) that is secured to a portion of the yoke 3, to output an opening signal in accordance with the valve positioner, that is, with the valve opening, of the flow rate regulating valve 1. This angle sensor 6 is structured from four magnetoresistive elements that are connected in a bridge, where an input voltage is applied to one of two opposing terminals, and the other set of two opposing terminals is used as output terminals.
7 is a feedback lever for inputting, into the angle sensor 6, opening information in accordance with the valve opening of the flow rate regulating valve 1, with one end thereof secured to the rotational shaft of the angle sensor 6. Moreover, a slit 7a is formed in this feedback lever 7, where the pin 5 engages slidably with this slit 7a, to convert the reciprocating motion of the valve driving shaft 4 into rotational motion.
FIG. 8 is a perspective diagram illustrating the critical portions of the angle sensor 6. The angle sensor 6 is provided with a magnetism detecting element 60, secured at a prescribed location, where magnets 61 and 62 are attached, facing each other, to a magnetic circuit forming body 63, with the magnetism detecting element 60 interposed therebetween. The center of the magnetic circuit forming body 63 is secured to one end of a rotational shaft 64. The other end of the rotational shaft 64 is secured to the feedback lever 7. When the rotational shaft 64 rotates in accordance with the shifting of the feedback lever 7, the magnets 61 and 62, together with the magnetic circuit forming body 63, rotate surrounding the magnetism detecting element 60, thereby changing the direction of the magnetic field that acts on the magnetism detecting element 60, thereby changing the value of the resistance of the magnetism detecting element 60.
FIG. 9 is a plan view diagram illustrating a structure for the magnetism detecting element 60. In this magnetism detecting element 60, four magnetoresistive elements (AMR elements (“anisotropic magnetoresistive” devices)) r1, r2, r3, and r4, which are shaped as zigzags, are formed on a substrate 60a with point symmetry so that the zigzag directions thereof are mutually perpendicular.
In this magnetism detecting element 60, when the magnetic field acts, for example, in the direction of arrow A, shown in FIG. 9, then the resistance values of the magnetoresistive elements r1 and r4, which are parallel thereto, will go to maximum values, and the resistance values of the magnetoresistive elements r2 and r3, which are perpendicular thereto, will go to minimum values. Moreover, when the magnetic field acts in the direction of the arrow B, illustrated in FIG. 9, the resistance values of the magnetoresistive elements r1 and r4 will go to minimum values, and the resistance values of the magnetoresistive elements r2 and r3 will go to maximum values.
In the magnetism detecting element 60, not only is the bridge circuit 65 structured through the four magnetoresistive elements r1, r2, r3, and r4 that are connected in a bridge, but also bridge midpoint electrical potentials V1 and V2 are obtained from the output terminals P3 and P4 are produced through the application of a constant current across the power supply terminals P1 and P2 of this bridge circuit 65.
In FIG. 7, 8 is the positioner main unit that is the valve opening controlling device. In this positioner main unit 8, compressed air is fed in from the outside, and a valve opening setting value θsp is sent through communication from a controller (not shown) that is located in a remote location. Moreover, the positioner main unit 8 reads in, as an opening signal that is in accordance with the current valve opening value θpv of the flow rate regulating valve 1, a difference V between the bridge midpoint electrical potentials V1 and V2 from the output terminals P3 and P4 (the bridge midpoint potential difference), along with performing the supply of electric power across the power supply terminals P1 and P2 of the angle sensor 6.
The positioner main unit 8 compares the current valve opening θpv of the flow rate regulating valve 1, read in by the angle sensor 6, to the valve opening setting value θsp, applied from the outside, to send, to the operating device 2, control air, generated from compressed air, in accordance with the comparison result, so that the valve driving shaft 4 is driven by the operating device 2, to perform control so that the valve position of the flow rate regulating valve 1 (the current valve opening value θpv) will match the valve opening setting value θsp.
In the flow rate regulating valve controlling system, if there is a large difference between the temperature of the fluid that flows through the flow rate regulating valve 1 and the temperature of the room, the temperature of the angle sensor 6 will be affected, through thermal conduction, by the flow rate regulating valve 1 via the yoke 3, producing a large difference from room temperature, producing variation in the output (the bridge midpoint potential difference V) of the angle sensor 6 through changes in the temperature characteristics of the magnetoresistive elements r1 through r4 that structure the bridge circuit 65.
Because of this, in the flow rate regulating valve controlling system set forth in, for example, Japanese Unexamined Patent Application Publication No. 2003-139561 (“the JP '561”), only the component that is dependent on temperature is detected based on the bridge midpoint electrical potentials V1 and V2 when a constant current is supplied to the bridge circuit 65 of the angle sensor 6, where the temperature of the angle sensor 6 (the ambient temperature) is acquired from the detected temperature-dependent component, where correction information corresponding to the temperature that has been acquired is used in performing temperature correction on the opening information (the angle information) obtained from the bridge midpoint potential difference V.
Specifically, in the JP '561, an addition result V1+V2 that is independent of the angle θ is calculated through adding the bridge midpoint electrical potentials V1 and V2 (the bridge midpoint electrical potential sum), to calculate the temperature of the bridge circuit 65 of the angle sensor 6 based on this addition result V1+V2 that is not dependent on the angle θ, to determine a correction value corresponding to the calculated temperature. This correction value is used in correcting when calculating the valve opening value θpv of the flow rate regulating valve 1 at the current time, to cancel in relation to the valve opening, produced depending on the temperature of the angle sensor 6.
Moreover, for example, Japanese Unexamined Patent Application Publication No. 2003-21503 (“the JP '503”) discloses a torque tube-type measuring instrument that uses an angle sensor. When the technology used in this torque tube-type measuring instrument is applied to the flow rate regulating valve controlling system described above, the total resistance of the magnetism detecting element 60 is detected based on the voltage across the power supply terminals P1 and P2 (the bridge power supply terminal voltage) when a constant current is applied to the bridge circuit 65 of the angle sensor 6, where the temperature of the angle sensor 6 is acquired based on the detected total resistance of the magnetism detecting element 60, where correction information corresponding to this acquired temperature is used to perform temperature correction on the opening information (the angle information) obtained from the bridge midpoint potential difference V.
Specifically, in the JP '503, a database for storing the empirically-derived relationship between the total resistance R of the magnetism detecting element 60 and the temperature is provided, where temperature data corresponding to the detected total resistance R of the magnetism detecting element 60 is read out from the database, and correction information that has been read out, corresponding to the temperature data, is used.
In the flow rate regulating valve controlling system set forth in the JP '561, the sum of the bridge midpoint electrical potentials V1 and V2 (the bridge midpoint electrical potential sum) from the angle sensor is used to detect the temperature of the angle sensor, so that, in the flow rate regulating valve controlling system to which the technology set forth in the JP '503 is applied, the voltage across the bridge power supply terminals of the angle sensor is used to detect the temperature of the angle sensor, that is, the temperature of the angle sensor is detected using the bridge midpoint electrical potential sum and the bridge power supply terminal voltage, which indicates the total resistance of the bridge circuit (the bridge total resistance), and thus there is no need to provide a separate temperature sensor, which has the benefit of enabling a simplification and a cost reduction in the structure and operability through reducing the number of components used.
However, the change in output of the angle sensor through the effects of temperature combines a zero point shift and a span shift. An illustrative example of a change in output of the angle sensor due to the effects of temperature is given in FIG. 10. In FIG. 10, the horizontal shaft is the angle θ(°), and the vertical shaft is the bridge midpoint potential difference V (mV), where X shows the case of an ambient temperature of 60° C., Y shows the case of an ambient temperature of 20° C., and Z shows the case of an ambient temperature of −20° C.
As illustrated in FIG. 10, the bridge midpoint potential difference V is scaled by the temperature (that is, the span is changed by the temperature), and also undergoes translational movement (that is, there is also a change in the zero point). All sensors that have bridge structures have such characteristics, where this is true also for a pressure sensor that is of a piezo type. Typically, the zero point shift and the span shift have different temperature characteristics, and so even though one may refer to simply “temperature correction,” it is actually necessary to model the characteristics of each independently, and to use respective correction calculation formulas.
Note that in the JP '561 and the JP '503, one may consider recording in advance sets of input angles, total resistances, and outputs (historic records), and interpolating points in between. In such a case there is no need to perform modeling; however there is a drawback in manufacturing in that this requires expensive equipment and an extensive amount of time to produce these “historic records.”
Moreover, in the JP '561 and the JP '503, one may consider the use of a calculation formula that models the span alone. In this case, only the span will be corrected, and the effects of the zero point shift, of course, will remain. Because of this, one may consider demanding, of the sensor vendor, a purchasing specification having favorable zero point characteristics to begin with (eliminating the need to consider the temperature shifts in the vicinity of the zero point to begin with), but “the “favorable characteristics” ultimately means “selecting through screening,” and because of this the manufacturability (the yield) will be poor, causing the cost of the angle sensor to be high. If zero point correction is possible, then excessive screening is unnecessary, enabling a reduction in the cost of the angle sensors.
The present invention was created in order to solve such issues, and an aspect thereof is to provide an angle sensor temperature correcting device able to provide a superior correction result at a low cost through eliminating the need for expensive equipment in creating the historic records and eliminating the need for excessive screening.